The immune system of humans and animals normally functions to protect its host from infectious organisms or from cancerous transformation by host cells. In many instances however, the immune system manifests a response that itself results in considerable damage to otherwise healthy cells and organs. Such over-reactivity of immune responsiveness is responsible for many serious conditions or diseases including allergies and autoimmune diseases.
In order to classify the processes by which the immune system produces cellular damage, immunologists have divided immune responses into four broad classes (Type I, II, III and IV) (Roitt, I. M., et al., Immunology, C. V. Mosby, N. Y., 1985, p. 19.1).
Type I responses are also called immediate hypersensitivity reactions and include those diseases which produce the symptoms classically associated with "allergies" or the "allergic syndrome" including allergic rhinitis (hay fever), allergic asthma, allergic conjunctivitis and allergic reactions to insect stings or foods. These conditions are characterized by a rapid clinical manifestation of allergic symptoms within minutes after exposure to an antigen (allergen) to which the subject has been previously sensitized.
In order for Type I hypersensitivity to occur, a specialized sequence of events within mast cells and basophils must be triggered by immunoglobulin E (IgE) antibodies that have been manufactured within the body. In this process, IgE directed toward an antigen (allergen) must bind to receptors on mast cells and basophils which specifically bind to the Fc region of IgE. Mast cells and basophils that have allergenspecific IgE bound to them are considered to be sensitized or "armed" for subsequent exposure to allergen. Should allergen be introduced into the local environment of the mast cells or basophils, the cells are automatically stimulated or "triggered" to release histamine and other vasoactive chemicals which produce the familiar "allergic symptoms" characteristic of allergic disease.
The hypersensitivity states characterized by types II, III and IV hypersensitivity are distinguished from type I hypersensitivity by many distinct and diverse features.
Type II hypersensitivity occurs when IgG or IgM antibodies bind to antigens located on the surfaces of cells. Such binding is mediated by the antibodies' Fab arms which contain specific structures that recognize cell surface antigens. Upon binding, the Fc regions of IgG or IgM interact with the complement system (a family of inflammatory and cell-killing molecules) or immune system "killer" cells bearing IgG or IgM Fc receptors. Some examples of diseases in which type II hypersensitivity reactions predominate include transfusion reactions, hemolytic disease of the newborn, autoimmune hemolytic anemias, hyperacute graft rejection, Goodpasture's syndrome, myesthenia gravis and other conditions.
Type III hypersensitivity is produced when complexes or aggregates of antibodies (usually IgG or IgM) and soluble antigens form in abnormally large amounts and activate the complement inflammatory system. Some examples of diseases in which type III hypersitivity reactions are pathogenically important include systemic lupus erythematosis, rheumatoid arthritis, polyarteritis and other forms of vasculitis, fibrosing alveolitis and many infectious diseases, especially bacterial endocarditis, hepatitis and malaria.
Type IV hypersensitivity (delayed-type hypersensitivity), by contrast to the other three hypersensitivity reactions, is triggered primarily by T cells having specialized T cell receptors able to recognize and bind to the specific sensitizing antigen on a cell's surface. Upon reexposure to an antigen, T cell receptor molecules bind to the antigen and trigger a complex series of events that result in secretion of lymphokines and other regulatory molecules that recruit new cells leading ultimately to the destruction of the antigen-bearing cell. Delayed type hypersensitivity, as its name implies, has a delayed onset of inflammation that ranges from about 24 hours to several days after reexposure to the sensitizing antigen. Diseases in which type IV hypersensitivity is believed to play an important pathogenic role are frequently termed "T-cell mediated" to reflect the unique role played by the T-cell in recognizing the sensitizing antigen. These diseases include multiple sclerosis, rheumatoid arthritis, juvenile onset diabetes mellitus, ulcerative colitis, and regional enteritis (Crohn's disease), among others.
An important principle that distinguishes type I hypersensitivity (allergy) from the other hypersensitivity states discussed above is that the allergic inflammation begins within minutes after allergen exposure. By contrast, other hypersensitivity states exhibit inflammation only after hours to days following reexposure to the sensitizing agent.
A second important principle that distinguishes type I hypersensitivity from other hypersensitivity states is the source of the sensitizing agent In type I hypersensitivity, the sensitizing agent (allergen) is not a part or component of the host body. Instead, the allergen is a substance found outside of the host body that is later introduced into the body by exposure to the environment. Types II, III and IV hypersensitivity, by contrast, may have immune responses directed towards antigens located on cells and molecules that are normal constituents of the body. Such immune responses toward normal constituents of the body are termed "autoimmune diseases" and constitute a medically important class of diseases distinct from allergic diseases.
A third important principle that distinguishes type I hypersensitivity from other hypersensitivity states is the degree to which cell killing occurs. In type I hypersensitivity, the IgE-mediated triggering reaction which causes the release of vasoactive allergic mediators does not result in the death of the releasing mast cell or basophil Instead, the "trigger" reaction is the result of an active secretory process that may recur after a length of time. Similarly, the effect of the vasoactive allergic mediators on surrounding cells is regulatory, not cytotoxic. Allergic mediators serve to increase the permeability of small blood vessels and activate a variety of vasoregulatory and immunoregulatory processes that do not normally result in cell death. Types II, III and IV hypersensitivity, by contrast, have as a principal function cell killing reactions which normally lead to the destruction of infectious agents or cancer cells.
In 1975, Hamburger reported that a pentapeptide with a sequence derived from the constant domain of human IgE could inhibit a local cutaneous allergic reaction (Prausnitz-Kustner) by approximately 90% (Hamburger, R., Science 189:389, 1975; U.S. Pat. Nos. 4,171,299 and 4,161,522). This pentapeptide, Asp-Ser-Asp-Pro-Arg, is known as Pentigetide The peptide has been shown to inhibit systemic allergic disease in humans after injection by the subcutaneous route.